The present invention relates to a fuel cell, such as a polymer electrolyte fuel cell, comprising an anode reactant supply plate and anode current collector and a cathode reactant supply plate and a cathode current collector. Further, the present invention relates to a method for determining the operational status of a fuel cell, such as a polymer electrolyte fuel cell, as given above.
The overall performance of a polymer electrolyte fuel cell (PEFC) depends on several factors: i) operating conditions, e.g. gas stoichiometry, temperature, humidification, operating pressure; ii) applied materials, e.g. type of membrane, catalysts, gas diffusion media (GDL) and iii) cell design, e.g. flow field design and orientation of the cell. Unfortunately, there is only limited insight into the influence of these factors, if an opinion is formed only by the measurement of the overall cell performance or by the application of diagnostic tools, like impedance spectroscopy (EIS) to the entire cell, since the active electrode area of PEFCs can exceed values of several tens of square centimetres for laboratory cells or even several hundreds of square centimetres for cells of technical scale.
Inhomogeneities in the local operating conditions due to variations in relative humidity, reactant concentration, temperature gradients, edge and mass transport effects or even manufacturing inconsistencies are highly likely to occur. This results in non-uniform performance over the active area.
When measuring over the entire active area of a PEFC, only average properties of cell parameters like current density or cell impedance can be determined. Thus, it is neither possible to identify inhomogeneities in fuel cell performance nor to get reliable information about limiting processes affecting the cell performance. Measurements performed over the entire active area of a PEFC are only meaningful if the local operating conditions are virtually homogenous throughout the cell, e.g. when using so called 1-dimensional cells.
Thus, improved diagnostic methods for PEFCs are badly needed not only for measuring but particularly for explaining inhomogeneities in cell performance. The application of these methods to an operating PEFC may provide more reliable and more meaningful data about the impact of factors like operating conditions or cell design on cell performance and the respective limiting processes.
To cope with this challenge, locally resolved current measurements in PEFCs may highlight inhomogeneities in the current distribution. However, by the application of locally resolved current measurements local inhomogeneities in the performance of PEFCs can only be identified but not explained. More advanced locally resolved diagnostic methods are needed to get information about the respective locally limiting processes and to gain a deeper understanding how operational parameters influence cell performance.
It is already known that there are three main processes limiting the performance of a PEFC, i.e. charge transfer, diffusion and ohmic resistance. The influence of these parameters on cell performance is highly dependent on the local operating conditions.
In principle Electrochemical Impedance Spectroscopy (EIS) can provide values for membrane resistance and charge transfer resistance as well as information about mass transport limitations. EIS is a powerful tool for in situ diagnostics in polymer electrolyte fuel cells. Thus, impedance spectroscopy has been applied to the entire cell before to get more insight about processes limiting cell performance.
But the information achieved by measuring the impedance spectrum of the entire cell is rather poor, since current density distribution measurements in PEFCs clearly show that inhomogeneities in local cell performance and thus in local cell impedance are highly likely to occur.
The main barrier to apply impedance spectroscopy in PEFCs in a locally resolved approach is measurement time. Time is a critical factor in EIS, since the cell can drift or a sudden change in the state of the cell can occur, i.e. at a given cell current (galvanostatic mode) or a given cell voltage (potentiostatic mode) the current density distribution can change. Depending on the frequency range the measurement of only one impedance spectrum can consume around half an hour of time which is not acceptable since the state of the cell can drift.
No local impedance measurements in realistic PEFCs of technical relevance and under realistic operating conditions have been reported so far.